Articles having a continuous grain size radial gradient and methods for making the same

ABSTRACT

An article is presented where the article comprises an alloy having a minor phase dispersed within a matrix phase and a plurality of substantially equiaxed grains. The article further comprises a continuous gradient in grain size from a first grain size at an outer surface of the article to a second grain size at an inner portion of the article, wherein the first grain size is less than the second grain size. Methods for forming the article using high deformation processing are also presented, where the processing includes extruding the feedstock material through a die having a twist channel configured to apply a torsional strain to the feedstock material as it passes through the die to form an extruded billet.

This application claims priority under 35 U.S.C. §119 to PatentApplication number A200613252, filed in the Ukraine on Dec. 14, 2006,the entirety of which is incorporated by reference herein.

BACKGROUND

The invention relates generally to high strength articles and methods ofmaking the same, and more specifically to articles having a continuousgradient in grain size and methods of making the same.

The continued effort to design and build more powerful and moreefficient turbo-machinery, such as gas turbines, steam turbines, andaircraft engines, requires the use of materials having enhancedperformance capabilities over a broad range of temperatures. Suchperformance enhancements require state-of-the-art materials withstability at high temperatures as well as improved mechanical propertiessuch as strength, fatigue resistance, and creep resistance.

Improved material strength is conventionally achieved in a number ofways including grain refinement, solid solution strengthening, compositestrengthening, or dispersoid strengthening. Strengthening alloys usinggrain refinement applies a mechanism referred to as Hall-Petchstrengthening. Hall-Petch strengthening relates to dislocation pile-upat grain boundaries and the stress required to propagate dislocationsacross grain boundaries. According to the Hall-Petch definition ofstrengthening, the strength of a material is inversely proportional tothe square root of the grain size. Grain refinement to nano-scale andsub-micron scale results in a large number of grain boundaries thatserve as barriers to dislocation motion, thus increasing the overallstrength of the material.

Manufactured articles often have competing requirements and in someapplications would benefit from a microstructure engineered to optimizelocal properties. For example, an airfoil in a gas turbinesimultaneously requires good resistance to thermal fatigue (surfacecracking associated with thermal cycling during operation) andresistance to creep (the elongation under steady state load at elevatedtemperatures). A fine grain size can provide high tensile strength andresistance to thermal fatigue. On the other hand, a coarse grain sizecan provide resistance to creep. Therefore, an airfoil is an example ofan article that may benefit from having an engineered structurecomprising both fine and coarse grains disposed in strategic regions onthe article.

Accordingly, there is a need in the art for improved high strengthmaterials and improved articles having a controlled variation in grainsize to enable better overall material performance.

BRIEF DESCRIPTION

Embodiments of the present invention meet these and other needs. Oneembodiment is an article comprising an alloy having a minor phasedispersed within a matrix phase and a plurality of substantiallyequiaxed grains. The article further comprises a continuous gradient ingrain size from a first grain size at an outer surface of the article toa second grain size at an inner portion of the article, wherein thefirst grain size is less than the second grain size.

Another embodiment is a method for forming an article. The methodcomprises providing a feedstock material comprising a minor phasedispersed within a matrix phase; and extruding the feedstock materialthrough a die having a twist channel configured to apply a torsionalstrain to the feedstock material as it passes through the die to form anextruded billet, wherein the extruding step is performed using apredetermined combination of temperature, strain, and strain rate suchthat a. during the extrusion step, the temperature of the feedstock ismaintained at below two thirds of the absolute melting temperature ofthe feedstock material, b. the feedstock is plastically deformed withoutsubstantially damaging the die, c. the feedstock material undergoessubstantially no recrystallization during the extruding step, and d. thefeedstock material undergoes substantially no dynamic recovery duringthe extruding step.

Another embodiment is a method for forming an article. The methodcomprises providing a feedstock material comprising a minor phasedispersed within a matrix phase; and extruding the feedstock materialthrough a die having a twist channel configured to apply a torsionalstrain to the feedstock material as it passes through the die to form anextruded billet, wherein the extruding step is performed using apredetermined combination of temperature, strain, and strain rate suchthat a. during the extrusion step, the feedstock temperature is in arange from about two thirds of the melting temperature of the feedstockmaterial to a solvus temperature of the minor phase, b. the feedstock isplastically deformed without substantially damaging the die, and c. thefeedstock material undergoes at least partial dynamic recrystallizationduring the extruding step.

DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is a cross-sectional side view of one embodiment of the instantinvention.

FIG. 2 is a flow chart of method steps associated with one method offabrication for the instant invention.

FIG. 3 is a perspective view of a twist extrusion die.

FIG. 4 is a table illustrating heat treatments and aging schedules forTi-6Al-4V billets tested using the twist extrusion method of the instantinvention.

DETAILED DESCRIPTION

An article 10 with a microstructure comprising a continuous gradient (G)in grain size from a first grain size at an outer surface 12 of thearticle 10 to a second grain size at an inner portion 14 of the article10, is shown in FIG. 1. As discussed above, grain size plays asignificant role in determining the mechanical properties of materials.An engineered article may benefit from a coarse grain size to provideone set of material characteristics, for example resistance to creep. Atthe same time, a finer grain size may be required to present a differentset of material characteristics, for example, to improve materialstrength. In some applications, an article may better meet desiredperformance requirements by having a graded microstructure ranging fromfiner grains to coarser grains. An airfoil for a gas or steam turbine,for example, simultaneously requires good resistance to thermal fatigueand resistance to creep, as discussed above.

While certain embodiments of this invention will be discussed in termsof an airfoil, this is not a limitation of the invention. In fact,article 10 may comprise any engineered article that would benefit from amicrostructure with a continuous grain size gradient from a first grainsize at an outer surface of the article to a second, larger grain sizeat an inner portion of the article. Turbine assembly components, such asfan blades, fan disks, compressor blades, compressor disks, airfoils,disks, blisks (bladed disks), ducts, frames, casings, hot gas pathcomponents, and the like are examples of components encompassed byembodiments of the present invention, as are other components such asshafts and gears.

Article 10 comprises an alloy. The alloy comprises a first phase, suchas, for example, a precipitate phase, that is dispersed within a matrixphase. In some embodiments, the first phase is substantially uniformlydispersed within the matrix phase. “Substantially uniformly” as usedherein means that the dispersion of first phase is sufficiently uniformsuch that the mechanical properties of the bulk material would not varysignificantly from those of a material having a completely uniformdispersion. Examples of alloys suitable for use in embodiments of thepresent invention include, but are not limited to, titanium alloys,superalloys, steels, aluminum alloys, copper alloys, magnesium alloys,refractory metal alloys, platinum-group metal alloys, and intermetallicalloys. One particular example of a titanium alloy is known in the artas Ti6-4, which has a nominal composition, in approximate weightpercent, of 6% Al, 4% V, balance Ti. Superalloys may includenickel-based superalloys, cobalt-based superalloys, iron-basedsuperalloys, and nickel-iron-based superalloys. One particular exampleof a nickel-iron based superalloy is known in the art as Alloy 718,which has a nominal composition, in approximate weight percent, of 18%Fe, 18% Cr, 3% Mo, 5% Nb, 1% Ti, 0.5% Al, 0.04% C, and 0.004% B, withthe balance being nickel (Ni).

Article 10 comprises a plurality of substantially equiaxed grains.“Substantially equiaxed” in this context means that the grains have axesapproximately equal in length, in contrast to grains typically observedin a casting, where the grains tend to have an elongated, columnarshape, especially near the outer surface of the casting.

Article 10 has a continuous gradient G in grain size from a first grainsize at an outer surface 12 of the article 10 to a second grain size atan inner portion 14 of the article. As used herein, “continuousgradient” is defined as a continuous proportional change in grain sizeover a given distance. The first grain size is smaller than the secondgrain size, so that article has a comparatively fine grain size at itsouter surface 12, and the grain size increases as a function of depthbeneath the outer surface 12. “Grain size” as used herein refers to themedian grain size at a particular depth as measured using standard grainsize measurement procedures, such as, for example, ASTM E112. In oneembodiment, the first grain size is in the range from about 10nanometers to about 1 micrometer. In another embodiment, the secondgrain size is in the range from about 1 micrometer to about 100micrometers. In particular embodiments, the article 10 comprises acontinuous gradient in grain size from a first grain size, in the rangefrom about 10 nanometers to about 1 micrometer, at outer surface 12 to asecond grain size, in the range from about 1 micrometer to about 100micrometers, at an inner portion of article 10. Articles having grainsizes within the ranges described above may offer several advantagesover articles with larger grain sizes, including, for example, higherstrength and the potential for superplastic formability.

One method 100 for fabricating article 10 with a microstructurecomprising a continuous grain size gradient (G) from a first grain sizeat an outer surface to a second grain size at a center portion is shownin FIG. 2. Generally, method 100 comprises providing 102 a feedstockmaterial, and subjecting 104 the feedstock material to an extremedeformation process, such as by twist-extruding the feedstock material.

In some embodiments, the providing step 102 comprises a pre-processingstep to provide the feedstock in a form suitable for use in the method100. Typically, bulk alloy materials, for example, alloys comprising oneor more of titanium (Ti), nickel (Ni), aluminum (Al), zirconium (Zr) andthe like, benefit from pre-processing to provide a billet of material asfeedstock for the extreme deformation in step 104. For example, if thealloy materials are provided as ingots or castings they are typicallyprocessed using a thermo-mechanical process such as extrusion, rolling,or forging, to create a billet. Standard thermo-mechanical processes arecommonly used in the art for various alloys to convert these alloys fromingots (which in highly alloyed systems often suffer from high degreesof chemical segregation) to billets having more uniform chemistry and/orstructure. In another embodiment, the starting feedstock is provided inthe form of powders produced by gas atomization or similar processes.The powder feedstock is processed, typically by hot isostatic pressingor extrusion, followed by thermo-mechanical processing to create abillet. In general, the process of creating a billet from startingmaterial such as ingots or feedstocks is called “billetizing” thematerial.

The feedstock thus provided comprises a first phase dispersed within amatrix phase. In certain embodiments, where the extreme deformation stepis performed within a temperature range where recrystallization of thematerial is likely to occur, this first phase may serve to pin grainboundaries to inhibit growth of the newly formed grains, therebyretaining a refined grain structure. The first phase is often a phasethat serves to strengthen the finished article. For example, innickel-based superalloys, the first phase may comprise the phase knownin the art as gamma prime, and in certain alloy systems such as Alloy718, the first phase may comprise the phase known as gamma double primeor the phase known as delta. In nickel based superalloy compositions,the first phase can be a gamma-prime precipitate phase. In titaniumalloys such as Ti-6Al-4V alloys, the first phase may be a grain-boundaryalpha phase or a reinforcement phase, such as TiB₂ particulates.

Next, the billet is introduced as feedstock into extreme deformationstep 104. Typically, the billet is subjected to extreme sheardeformation in a twist die in a process called twist extrusion. Twistextrusion involves extrusion of billets through a die 300 having a twistchannel 310, as schematically shown in FIG. 3. The die 300 is thusconfigured to apply a torsional strain to the feedstock material as itpasses through the die to form an extruded billet. The dimensions of thecross-section of the twist channel 310 that is orthogonal to theextrusion axis 320 is constant. A twist die typically has a die angle(y) in the range between about 30° to about 60°. Typically, a work pieceis pushed through the twist die using a piston or the like. The twistextrusion process can include a single pass or multiple passes throughthe twist die 300 and may be performed cold or with added heat. Duringeach pass through the twist die 300, the billet is subjected to extremeshear deformation, resulting in the generation of extremely finedeformation substructure and significant grain refinement. In oneembodiment, after an initial pass through the twist die, the billet isrotated, outside of the twist die, prior to subsequent passes throughthe twist die. The billet can be rotated clockwise or counterclockwiseoutside of the twist die at a variety of angles, for example at apredetermined angle in the range between about 10° to about 90°. Theextent of the extreme deformation may be controlled during this process,as discussed in greater detail below, such that nanoscale to sub-micronscale refined structures can be produced. The extreme deformation step104 can work with a variety of materials, including, for example,titanium alloys; nickel, iron and cobalt based superalloys; steels;aluminum alloys; copper alloys; magnesium alloys; refractory metalalloys, including tungsten, molybdenum, tantalum and niobium and theiralloys; platinum group metal alloys; and intermetallic alloys. In short,the feedstock may be any material described above for the articleembodiments.

In order to accommodate such a wide variety of materials, deformationstep 104 is typically applied according to certain parameters that varyin accordance with the material being processed. In one embodiment, thetemperature of the feedstock in the deformation process is maintainedbelow about two-thirds of the absolute melting temperature (that is, thetemperature as measured using an absolute scale, such as the Kelvinscale) of the feedstock material. This embodiment may be referred toherein as the “low-temperature regime.” In another embodiment, thetemperature of the feedstock in the deformation process is in a rangefrom about two-thirds of the absolute melting temperature of thefeedstock material to about the solvus temperature of the first phase.This embodiment may be referred to herein as the “high temperatureregime.”

Regardless of whether the method is being performed in the hightemperature regime or the low temperature regime, the temperature atwhich twist extrusion is performed is selected to be high enough thatthe flow stress of the material being twist extruded is substantiallybelow that of the material being used as the tooling. The flow stressdepends on the material being used, including its composition andmetallurgical condition, the temperature of that material, the strainbeing applied, and the strain-rate at which the billet is beingstrained. The flow stress ratio, defined by the compressive yieldstrength of the tool material divided by the flow stress of the billetmaterial, is a good indicator of success. If this flow stress ratio isless than one, then the tooling will likely deform about the billet, orthe billet will not move through the die or tool. If the flow stressratio is greater than one, and typically greater than 3, and oftengreater than 5, the tool material is likely to survive the twistextrusion process. Table 1 indicates flow stress ratios of a variety ofmaterials at certain twist extrusion conditions.

TABLE 1 Billet Tool Twist Flow Billet Temperature Tool Temperature AnglePeak Peak Strain Stress Material (° C.) Material (° C.) (g) Strain rate(1/s) Ratio Ti—6Al—4V 800 AISI H13 300 30 0.39 1.00 4.80 Ti—6Al—4V 900AISI H13 300 30 0.39 1.00 9.14 Ti—6AI—4V 1000 AISI H13 300 30 0.39 1.0031.37 Ti—6Al—4V 800 Ni-718 600 30 0.39 1.00 2.70 Ti—6Al—4V 900 Ni-718600 30 0.39 1.00 5.14 Ti—6Al—4V 1000 Ni-718 600 30 0.39 1.00 17.65Ti—6Al—4V 800 TZM 800 30 0.39 1.00 1.50 Ti—6Al—4V 900 TZM 800 30 0.391.00 2.86 Ti—6Al—4V 1000 TZM 800 30 0.39 1.00 9.80 Ni-718 (ST) 900 AISIH13 300 30 0.39 1.00 2.96 Ni-718 (ST) 1000 AISI H13 300 30 0.39 1.005.37 Ni-718 (ST) 1100 AISI H13 300 30 0.39 1.00 8.12 Ni-718 (ST) 900Ni-718 600 30 0.39 1.00 1.67 Ni-718 (ST) 1000 Ni-718 600 30 0.39 1.003.02 Ni-718 (ST) 1100 Ni-718 600 30 0.39 1.00 4.57 Ni-718 (ST) 900 TZM800 30 0.39 1.00 0.93 Ni-718 (ST) 1000 TZM 800 30 0.39 1.00 1.68 Ni-718(ST) 1100 TZM 800 30 0.39 1.00 2.54 304L SS 800 AISI H13 300 30 0.391.00 4.30 304L SS 1000 AISI H13 300 30 0.39 1.00 7.96 304L SS 1200 AISIH13 300 30 0.39 1.00 19.05 304L SS 800 Ni-718 600 30 0.39 1.00 2.42 304LSS 1000 Ni-718 600 30 0.39 1.00 4.48 304L SS 1200 Ni-718 600 30 0.391.00 10.71 304L SS 800 TZM 800 30 0.39 1.00 1.34 304L SS 1000 TZM 800 300.39 1.00 2.49 304L SS 1200 TZM 800 30 0.39 1.00 5.95

In the low-temperature regime, the twist extrusion process is performedusing a predetermined combination of temperature, strain and strain ratesuch that a. during the extrusion step, the temperature of the feedstockis maintained at below two thirds of the absolute melting temperature ofthe feedstock material, b. the feedstock is plastically deformed withoutsubstantially damaging the die, c. the feedstock material undergoessubstantially no recrystallization during the extruding step, and d. thefeedstock material undergoes substantially no dynamic recovery duringthe extruding step.

Factors a. and b. above represent a balance in the selection of, amongother things, the temperature in this regime. The temperature issufficiently high to avoid damage to the tooling, but sufficiently lowto allow for significant deformation to occur without risk ofuncontrolled grain growth. In certain embodiments, such as where thefeedstock is a superalloy such as Alloy 718, the temperature of thefeedstock is less than about 725° C.; this temperature may be less thanabout 625° C. in particular embodiments, such as where the feedstock isa titanium alloy such as Ti-6-4. Where the temperature is kept in thislow temperature regime, the presence of the first phase is typically notrequired to maintain grain size because there is not sufficient thermalenergy to drive significant grain growth. Accordingly, in someembodiments, the first phase is dissolved into the matrix (generally bya solutionizing heat treatment, commonly performed in the art) to form asingle-phase material prior to extruding. The dissolution of the firstphase may lower the flow stress of the feedstock, thereby allowing thefeedstock to pass more easily through the die at these relatively lowtemperatures.

Strain introduced into the feedstock during twist extrusion is afunction of the angle of die twist and the number of passes through thedie to which the feedstock is subjected. The strain that is referred toherein represents the strain measured at an outermost surface of thefeedstock material, where strain is at a maximum due to the geometry ofthe die. In some embodiments, the strain is greater than about 0.2, andin particular embodiments, the strain is greater than about 0.4.Generally, high strain correlates to a high driving force for thenucleation of recrystallized grains, and so a comparatively high amountof strain is typically employed where a fine grain size is desired.

The strain rate is generally determined by how quickly the feedstock isforced through the twist die. In the low temperature regime, asdescribed above, no substantial amount of recrystallization or recoveryis permitted to occur during the extrusion step. Accordingly, the strainrates can be relatively fast compared to other regimes where dynamicrecrystallization or recovery processes are desired. The latter regimesrequire slower deformation rates to allow for newly formed dislocationsto move and form, for example, into desired sub-structures; the formerrequires no such consideration. Of course, the strain rate should not beso high as to cause undue transient increases in local flow stresssufficient to damage the die. In certain embodiments, the strain rate isin the range from about 0.1 sec⁻¹ to about 0.5 sec⁻¹.

In the high temperature regime, the twist extrusion step is performedusing a predetermined combination of temperature, strain, and strainrate such that a. during the extrusion step, the feedstock temperatureis in a range from about two thirds of the melting temperature of thefeedstock material to a solvus temperature of the first phase, b. thefeedstock is plastically deformed without substantially damaging thedie, and c. the feedstock material undergoes at least partial dynamicrecrystallization during the extruding step.

In this regime, where the temperature is sufficiently high thatrecrystallization and grain growth during processing may come into play,the presence of the first phase during the extrusion step serves as aninhibitor to undesirably uncontrolled grain growth. In certainembodiments, such as where the feedstock is a titanium alloy such asTi-6-4, the temperature of the feedstock is at least about 625° C.; thistemperature may be at least about 725° C. in particular embodiments,such as where the feedstock is a superalloy such as Alloy 718. Thetemperature is typically bound on the upper end by the solvustemperature of the first phase, to ensure that sufficient first phase ispresent to pin the grains of the feedstock during processing. Fortitanium alloys, for example, this upper temperature limit is typicallyequal to or less than the beta transus temperature, for example betweenabout 750° C. to about 850° C. For nickel-iron alloy systems such asAlloy 718, this upper temperature limit is typically below about 1000°C.

In some embodiments, the amount of strain introduced into the feedstockmaterial during twist extrusion is the same as described above for thelow temperature regime. The rate at which strain is applied in the hightemperature regime, however, may differ somewhat from that in the lowtemperature regime, because in the high temperature regime the strainrate is sufficiently low to allow for at least partial dynamicrecrystallization to occur during the extrusion step. As used herein,“partial dynamic recrystallization” means a portion of themicrostructure undergoes recrystallization. In certain embodiments, thestrain rate is in the range from about 10⁻⁴ sec⁻¹ to about 10⁻² sec⁻¹.

Regardless of which of the above regimes is used, a post-processing step106 is applied in some embodiments to provide further propertyenhancements to the extruded billet or to otherwise form the billet intoa desired shape. The post-processing step generally comprises one ormore processes used in the metal processing art. Examples of suchprocesses include, but are not limited to, extrusion (such as byconventional extrusion methods), rolling, forging, and heat treating. Insome embodiments, one of the primary functions of the post-processingstep is to cause recrystallization of grains within the extruded billet.For example, heat treatment or other elevated-temperature processesoperated above about two-thirds of the melting temperature (absolutescale) of the material making up the extruded billet is likely totrigger recrystallization where a sufficient driving force (such asretained strain energy from the twist extrusion step) exists within theextruded billet. Generally the post-processing is performed below asolvus temperature for the first phase to avoid uncontrolled graingrowth. In certain embodiments, the grain size of the extruded billetmaterial is controlled to provide a balance of desirable properties, asdescribed previously. Fine-grained materials are typically desired forsuperior strength, for example. In some embodiments, afterpost-processing, the grains have a median grain size less than about 10micrometers.

As described previously, the article produced by the above method canhave a gradient G in grain size. Where twist extrusion is performed inaccordance with embodiments of the present invention, the outer surfaceof the extruded billet experiences the highest amount of strain, whilethe centerline of the billet may experience very little strain. As aresult, the driving force for recrystallization, and hence thenucleation rate of new grains, is comparatively low at an inner portionof the extruded billet and comparatively high at the outer portion, witha continuous gradient in existing between these points as dictated bythe geometry of the die. Where nucleation rates are high, grain sizetends to be low when processed according to embodiments of the presentinvention. The result is that grains are disposed within the extrudedbillet such that the extruded billet has a microstructure comprising acontinuous gradient (G, FIG. 1) in grain size, ranging from a firstgrain size at the outer surface 12 to a second grain size at an innerportion 14. Where the material is processed according to embodimentsdescribed herein, the first grain size is typically less than the secondgrain size. As stated above, in one embodiment, the first grain size isin the range from about 10 nanometers to about 1 micrometer. In anotherembodiment, the second grain size is in the range from about 1micrometer to about 100 micrometers. In particular embodiments, thearticle 10 comprises a continuous gradient in grain size from a firstgrain size, in the range from about 10 nanometers to about 1 micrometer,at outer surface 12 to a second grain size, in the range from about 1micrometer to about 100 micrometers, at an inner portion of article 10.

EXAMPLES

The following examples are provided to further illustrate particularembodiments of the invention described above; they should not beconstrued as limiting the invention in any way.

Example 1

A feedstock material comprising a nickel-based superalloy or anickel-iron-based superalloy is provided, and is twist extruded suchthat the temperature of the feedstock is less than about 725° C. The dieused for the extrusion is shaped such that the strain, as measured bythe strain at an outermost surface of the feedstock material, is greaterthan about 0.2. The rate at which the feedstock is forced through thedie is controlled so that the strain rate is in the range from about 0.1sec⁻¹ to about 0.5 sec⁻¹. The extruded billet thus formed may besubjected to a number of repetitions of the twist extrusion step, toincrease the total strain accumulated within the material. After twistextrusion, the extruded billet is post-processed to causerecrystallization of grains within the extruded billet.

Example 2

A feedstock material comprising a titanium alloy is provided, and istwist extruded such that the temperature of the feedstock is less thanabout 625° C. The die used for the extrusion is shaped such that thestrain, as measured by the strain at an outermost surface of thefeedstock material, is greater than about 0.2. The rate at which thefeedstock is forced through the die is controlled so that the strainrate is in the range from about 0.1 sec⁻¹ to about 0.5 sec⁻¹. Theextruded billet thus formed may be subjected to a number of repetitionsof the twist extrusion step, to increase the total strain accumulatedwithin the material. After twist extrusion, the extruded billet ispost-processed to cause recrystallization of grains within the extrudedbillet.

Example 3

A feedstock material comprising a nickel-based superalloy or anickel-iron-based superalloy is provided, and is twist extruded suchthat the temperature of the feedstock is in the range from about 725° C.to about 1000° C. The die used for the extrusion is shaped such that thestrain, as measured by the strain at an outermost surface of thefeedstock material, is greater than about 0.2. The rate at which thefeedstock is forced through the die is controlled so that the strainrate is in the range from about 10⁻⁴ sec⁻¹ to about 10⁻² sec⁻¹. Theextruded billet thus formed may be subjected to a number of repetitionsof the twist extrusion step, to increase the total strain accumulatedwithin the material. After twist extrusion, the extruded billet ispost-processed to cause recrystallization of grains within the extrudedbillet.

Example 4

A feedstock material comprising a titanium alloy is provided, and istwist extruded such that the temperature of the feedstock is in therange from about 625° C. to about 1000° C. The die used for theextrusion is shaped such that the strain, as measured by the strain atan outermost surface of the feedstock material, is greater than about0.2. The rate at which the feedstock is forced through the die iscontrolled so that the strain rate is in the range from about 10⁻⁴ sec⁻¹to about 10⁻² sec⁻¹. The extruded billet thus formed may be subjected toa number of repetitions of the twist extrusion step, to increase thetotal strain accumulated within the material. After twist extrusion, theextruded billet is post-processed to cause recrystallization of grainswithin the extruded billet.

Example 5

Hot rolled Ti-6Al-4V and forged nickel Alloy 718 billets were tested asprototype Ti and Ni-structured alloys, respectively. Twist dies having arectangular cross-section of 15×25 mm and with 3 different twist angles(30°, 45°, and 60°) were selected for the extrusion process. Prior tothe twist extrusion, to achieve a baseline microstructure, the feedstockpieces were heated as follows: Ti-6Al-4V was heat treated at 1000° C.for 1 hour and air cooled; Alloy 718 was heat treated at 1010° C. for 1hour and air cooled. An auxiliary billet (Zn, Cu or commercially pureTi) was attached to the front of the feedstock during the twistextrusion process in order to exert a counter-pressure during thedeformation. The length and the choice of the auxiliary billet materialcontrolled the level of counter pressure. The use of thecounter-pressure billet allows the creation of additional pressure of upto about 1000 MPa. Without the auxiliary billet, the twist deformation,under certain conditions, can result in the formation of microvoids atgrain boundary triple points, or even the fracture of the workpiece. TheTi-6Al-4V and Alloy 718 billets were warm extruded by using the twistextrusion process at temperatures in the range between about 500° C. toabout 600° C. to avoid dynamic recrystallization. Subsequent heattreatments were then applied to the twist extruded Ti-6Al-4V material tofurther refine the substructure or grains in the material and place thematerial in a condition for use in high strength or high temperaturestructural or rotating components in steam turbines, gas turbines,aircraft engines or other hot gas applications. The twist extrudedTi-6Al-4V material was heat treated and aged according to the schedulein FIG. 4. The resulting material had a significantly finer grainstructure than that of the starting material, and a clearly observablegradient in grain size with finer grain sizes at the outer surface thatbecame more coarse with depth below the outer surface.

As discussed above, the continued effort to design and build morepowerful and more efficient turbo-machinery, such as gas turbines, steamturbines, and aircraft engines, requires the use of materials havingenhanced performance capabilities over a range of temperatures. Thesecomponents include rotating components, such as fan blades, fan disks,compressor blades, compressor disks, turbine airfoils, blisks, disks,and fixed components, such as ducts, frames, casings, hot gas pathcomponents, and airframes. In method 100, bulk metal alloys areprocessed and subsequently subjected to extreme shear deformation tocreate a structural material with improved mechanical propertiescompared to conventional alloys. In addition, method 100 reducesprocessing inhomogeneities and defects that are commonplace in cast andpowder metallurgy components. In fact, the resulting high strength,fine-grained billet material can be used in a variety of applications,over an extremely broad range of temperatures depending on the materialsystem, for example from about 100° C. to about 700° C. for nickelalloys. Applications at the high end of the temperature range caninclude, as discussed above, high strength structural or rotatingcomponents in steam turbines, gas turbines, aircraft engines and otherhigh temperature applications.

While only certain features of the invention have been illustrated anddescribed herein, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

1. A method for forming an article, the method comprising: providing afeedstock material comprising a first phase dispersed within a matrixphase; and extruding the feedstock material through a die having a twistchannel configured to apply a torsional strain to the feedstock materialas it passes through the die to form an extruded billet, wherein theextruding step is performed using a predetermined combination oftemperature, strain, and strain rate such that a. during the extrusionstep, the temperature of the feedstock is maintained at below two thirdsof the absolute melting temperature of the feedstock material, b. thefeedstock is plastically deformed without substantially damaging thedie, c. the feedstock material undergoes substantially norecrystallization during the extruding step, and d. the feedstockmaterial undergoes substantially no dynamic recovery during theextruding step.
 2. The method of claim 1, further comprisingpost-processing the extruded billet using at least one process selectedfrom the group consisting of extrusion, rolling, forging, heat treating,and combinations thereof.
 3. The method of claim 2, wherein thepost-processing causes recrystallization of grains within the extrudedbillet.
 4. The method of claim 3, wherein, after the post-processing,the grains have a median grain size less than about 10 micrometers. 5.The method of claim 1, further comprising dissolving the first phaseinto the matrix to form a single phase material prior to extruding. 6.The method of claim 1, wherein the grains are disposed within theextruded billet such that the extruded billet has a microstructurecomprising a continuous size gradient from a first grain size at anouter surface of the extruded billet to a second grain size at an innerportion of the extruded billet.
 7. The method of claim 6, wherein thefirst grain size is less than the second grain size.
 8. The method ofclaim 7, wherein the first grain size is in the range from about 10nanometers to about 1 micrometer.
 9. The method of claim 7, wherein thesecond grain size is in the range from about 1 micrometer to about 100micrometers.
 10. The method of claim 1, wherein the feedstock materialcomprises at least one alloy selected from the group consisting oftitanium alloys, superalloys, steels, aluminum alloys, copper alloys,magnesium alloys, refractory metal alloys, platinum-group metal alloys,and intermetallic alloys.
 11. The method of claim 10, wherein thefeedstock material comprises a superalloy selected from the groupconsisting of nickel-based superalloys, cobalt-based superalloys,iron-based superalloys, and nickel-iron-based superalloys.
 12. Themethod of claim 1, wherein the feedstock material comprises Alloy 718.13. The method of claim 1, wherein the feedstock material comprisesTi-6% Al-4% V alloy.
 14. The method of claim 1, wherein the temperatureof the feedstock material is less than about 725° C.
 15. The method ofclaim 14, wherein temperature of the feedstock material is less thanabout 625° C.
 16. The method of claim 1, wherein the strain, as measuredby the strain at an outermost surface of the feedstock material, isgreater than about 0.2.
 17. The method of claim 16, wherein the strainis greater than about 0.4.
 18. The method of claim 1, wherein the strainrate is in the range from about 0.1 sec⁻¹ to about 0.5 sec⁻¹.
 19. Amethod for forming an article, the method comprising: providing afeedstock material comprising a nickel-based superalloy or anickel-iron-based superalloy; extruding the feedstock material through adie having a twist channel configured to apply a torsional strain to thefeedstock material as it passes through the die to form an extrudedbillet, wherein the extruding step is performed using a predeterminedcombination of temperature, strain, and strain rate such that a. thetemperature of the feedstock material is less than about 725° C., b. thestrain, as measured by the strain at an outermost surface of thefeedstock material, is greater than about 0.2, and c. the strain rate isin the range from about 0.1 sec⁻¹ to about 0.5 sec⁻¹; andpost-processing the extruded billet to cause recrystallization of grainswithin the extruded billet.
 20. A method for forming an article, themethod comprising: providing a feedstock material comprising a titaniumalloy; extruding the feedstock material through a die having a twistchannel configured to apply a torsional strain to the feedstock materialas it passes through the die to form an extruded billet, wherein theextruding step is performed using a predetermined combination oftemperature, strain, and strain rate such that a. the temperature of thefeedstock material is less than about 625° C., b. the strain, asmeasured by the strain at an outermost surface of the feedstockmaterial, is greater than about 0.2, and c. the strain rate is in therange from about 0.1 sec⁻¹ to about 0.5 sec⁻¹; and post-processing theextruded billet to cause recrystallization of grains within the extrudedbillet.
 21. A method for forming an article, the method comprising:providing a feedstock material comprising a first phase dispersed withina matrix phase; and extruding the feedstock material through a diehaving a twist channel configured to apply a torsional strain to thefeedstock material as it passes through the die to form an extrudedbillet, wherein the extruding step is performed using a predeterminedcombination of temperature, strain, and strain rate such that a. duringthe extrusion step, the feedstock temperature is in a range from abouttwo thirds of the melting temperature of the feedstock material to asolvus temperature of the first phase, b. the feedstock is plasticallydeformed without substantially damaging the die, and c. the feedstockmaterial undergoes at least partial dynamic recrystallization during theextruding step.
 22. The method of claim 21, further comprisingpost-processing the extruded billet using at least one process selectedfrom the group consisting of extrusion, rolling, forging, heat treating,and combinations thereof.
 23. The method of claim 22, wherein thepost-processing causes recrystallization of grains within the extrudedbillet.
 24. The method of claim 22, wherein, after the post-processing,the grains have a median grain size less than about 10 micrometers. 25.The method of claim 23, wherein the grains are disposed within theextruded billet such that the extruded billet has a microstructurecomprising a continuous size gradient from a first grain size at anouter surface of the extruded billet to a second grain size at a centerportion of the extruded billet.
 26. The method of claim 25 wherein thefirst grain size is less than the second grain size.
 27. The method ofclaim 26 wherein the first grain size is in the range from about 10nanometers to about 1 micrometer.
 28. The method of claim 26 wherein thefirst grain size is in the range from about 1 micrometer to about 100micrometers.
 29. The method of claim 21 wherein the feedstock materialcomprises at least one alloy selected from the group consisting oftitanium alloys, superalloys, steels, aluminum alloys, copper alloys,magnesium alloys, refractory metal alloys, platinum-group metal alloys,and intermetallic alloys.
 30. The method of claim 29, wherein thefeedstock material comprises a superalloy selected from the groupconsisting of nickel-based superalloys, cobalt-based superalloys,iron-based superalloys, and nickel-iron-based superalloys.
 31. Themethod of claim 21, wherein the feedstock material comprises Alloy 718.32. The method of claim 21, wherein the feedstock material comprisesTi-6% Al-4% V alloy.
 33. The method of claim 21, wherein the temperatureof the feedstock material is at least about 725° C.
 34. The method ofclaim 21, wherein temperature of the feedstock material is at leastabout 625° C.
 35. The method of claim 21, wherein the strain, asmeasured by the strain at an outermost surface of the feedstockmaterial, is at least about 0.2.
 36. The method of claim 35, wherein thestrain is at least about 0.4.
 37. The method of claim 21, wherein thestrain rate is in the range from about 10⁻⁴ sec⁻¹ to about 10⁻² sec¹.38. A method for forming an article, the method comprising: providing afeedstock material comprising a nickel-based superalloy or anickel-iron-based superalloy; extruding the feedstock material through adie having a twist channel configured to apply a torsional strain to thefeedstock material as it passes through the die to form an extrudedbillet, wherein the extruding step is performed using a predeterminedcombination of temperature, strain, and strain rate such that a. thetemperature of the feedstock material is in the range from about 725° C.to about 1000° C., b. the strain, as measured by the strain at anoutermost surface of the feedstock material, is at least about 0.2, andc. the strain rate is at least about 10⁻² sec⁻¹; and post-processing theextruded billet to cause recrystallization of grains within the extrudedbillet.
 39. A method for forming an article, the method comprising:providing a feedstock material comprising a titanium alloy; extrudingthe feedstock material through a die having a twist channel configuredto apply a torsional strain to the feedstock material as it passesthrough the die to form an extruded billet, wherein the extruding stepis performed using a predetermined combination of temperature, strain,and strain rate such that a. the temperature of the feedstock materialis in the range from about 625° C. to about 1000° C., b. the strain, asmeasured by the strain at an outermost surface of the feedstockmaterial, is at least about 0.2, and c. the strain rate is at leastabout 10⁻² sec⁻¹; and post-processing the extruded billet to causerecrystallization of grains within the extruded billet.
 40. An articlecomprising: an alloy comprising a first phase dispersed within a matrixphase; a plurality of substantially equiaxed grains; and a continuousgradient in grain size from a first grain size at an outer surface ofthe article to a second grain size at an inner portion of the article,wherein the first grain size is less than the second grain size.
 41. Thearticle of claim 40, wherein the first phase is substantially uniformlydispersed within the matrix phase.
 42. The article of claim 40, whereinthe first grain size is in the range from about 10 nanometers to about 1micrometer.
 43. The article of claim 40, wherein the second grain sizeis in the range from about 1 micrometer to about 100 micrometers. 44.The article of claim 40, wherein the alloy comprises at least oneselected from the group consisting of titanium alloys, superalloys,steels, aluminum alloys, copper alloys, magnesium alloys, refractorymetal alloys, platinum-group metal alloys, and intermetallic alloys. 45.The article of claim 44, wherein the alloy comprises a superalloyselected from the group consisting of nickel-based superalloys,cobalt-based superalloys, iron-based superalloys, and nickel-iron-basedsuperalloys.
 46. The article of claim 45, wherein the alloy comprisesAlloy
 718. 47. The article of claim 44, wherein the alloy comprisesTi-6% Al-4% V alloy.
 48. The article of claim 40, wherein the articlecomprises a component of a turbine assembly.
 49. The article of claim48, wherein the component is selected from the group consisting of a fanblade, a fan disk, a compressor blade, a compressor disk, a turbineairfoil, a disk, a duct, a frame, a casing, and a hot gas pathcomponent.
 50. The article of claim 50, wherein the article comprises ashaft or a gear.